1) The RR is for the Spirit to resist being trapped.
2) The bonus is for items that cause physical attacks (i.e. using an elemental fire spirit to do a tiny bolt attack). And don't forget that the additional spirits can EITHER increase the bonus of such attack, or to alter/improve the attack, so it would require a lot of spirits to shift things.
3) And finally, don't forget that the spirit devices are one-shot devices. Once used, the spirits are all gone, released back into the Spirit World (which kind of automatically prevents them from being trapped again, and more would have to be captured, and THAT can be a long and arduous process.